1. Field of the Invention:
This invention relates to a hydraulic control apparatus including poppet valves, and more particularly to such a hydraulic control apparatus for controlling a plurality of actuators such as hydraulic cylinder means adapted to drive work implements mounted on heavy excavators and cranes.
2. Description of the Prior Art:
In the past, there has been employed a hydraulic control apparatus of the kind specified which comprises at least two sets of meter-in poppets and at least two sets of meter-out poppets mounted in a valve body in such a manner that their outer peripheral sides are located in their primary ports and their leading ends are located in their secondary port. Each of the poppets is arranged to be opened and shut freely through a pilot valve adapted to be actuated by the control signal generated and transmitted by a controller in proportion to the manipulated variable produced by a group of operating levers. The primary side ports of the meter-in poppets is connected to a variable displacement pump and their secondary ports are respectively connected through respective load check valves to the primary side ports of said meter-out poppets and at least one actuator. The pilot piston slidably mounted in each of the pilot valves is formed with a passage which is arranged, at neutral position thereof, to release the fluid pressure in the port of the meter-out poppet valve that corresponds to the control signal.
However, such a hydraulic control apparatus tends to become large-sized and expensive in manufacturing cost without proper arrangements of the poppet valves and pilot valves.
Further, the hydraulic control systems disclosed in U.S. Pat. Nos. 4,201,052 and 4,535,809 are not suitable for use as those which require hydraulic fluid to flow therethrough at high flow rates in hydraulic cylinders mounted in large-sized excavators.